A new poll of Republican candidates seeking their party's presidential nomination shows frontrunners Donald Trump and Ben Carson with strong leads, and former technology executive Carly Fiorina tied for third place with Florida Senator Marco Rubio.

On the Democratic side, the NBC News-Wall Street Journal poll shows frontrunner Hillary Clinton continuing to lose ground to left-leaning Bernie Sanders, with her earlier double-digit lead now cut to seven points, at 42-35 percent.

The poll shows real estate tycoon Trump with a slight 21-20 percent lead over the soft-spoken Carson. In July, Trump led with 19 percent to Carson's 10 percent.

Fiorina and Rubio are tied at 11 percent, followed by Jeb Bush (7 percent), John Kasich (6) and Ted Cruz (5).

Separately, the socially conservative Family Research Council Action, a Christian lobbying group, said attendees at a Values Voter Summit last week showed strong support for Cruz, the right-wing Texas senator and leading voice of the Republican tea party movement.

In a statement, the group said Cruz held the support of 35 percent of those polled, while retired neurosurgeon Carson was supported by 18 percent, followed by Mike Huckabee with 14 percent.

A potential White House ticket of Trump with Huckabee as vice presidential running mate was backed by 5 percent.